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The Impact of Stripped Nuclei on the Supermassive Black Hole Number Density in the Local Universe

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The Impact of Stripped Nuclei on the Supermassive Black Hole Number Density in the Local Universe The Astrophysical Journal, 871:159 (11pp), 2019 February 1 https://doi.org/10.3847/1538-4357/aaf735 © 2019. The American Astronomical Society. All rights reserved. The Impact of Stripped Nuclei on the Supermassive Black Hole Number Density in the Local Universe Karina T. Voggel1 , Anil C. Seth1, Holger Baumgardt2, Steffen Mieske3, Joel Pfeffer4 , and Alexander Rasskazov5 1 University of Utah, James Fletcher Building, 115 1400 E, Salt Lake City, UT 84112, USA; [email protected] 2 School of Mathematics and Physics, University of Queensland, St. Lucia, QLD 4072, Australia 3 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile 4 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 5 School of Physics and Astronomy and Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623, USA Received 2018 October 9; revised 2018 December 5; accepted 2018 December 8; published 2019 January 29 Abstract The recent discovery of supermassive black holes (SMBHs) in high mass ultra-compact dwarf galaxies (UCDs) suggests that at least some UCDs are the nuclear star clusters of stripped galaxies. In this paper we present a new method to estimate how many UCDs host an SMBH and thus are stripped galaxy nuclei. We revisit the dynamical mass measurements that suggest many UCDs have more mass than expected from stellar population estimates, which observations have shown is due to the presence of an SMBH. We revise the stellar population mass estimates using a new empirical relation between the mass-to-light ratio (M/L) and metallicity to predict which UCDs most likely host an SMBH. We calculate the fraction of UCDs that host SMBHs across their entire luminosity range for the first time. We then apply the SMBH occupation fraction to the observed luminosity +32 function of UCDs and estimate that in the Fornax and Virgo clusters alone there should be 69-25 stripped nuclei with SMBHs. This analysis shows that stripped nuclei are almost as common in clusters as present-day galaxy nuclei. We estimate the SMBH number density caused by stripped nuclei to be (2–8)×10−3 Mpc−3, which represents a significant fraction (8%–32%) of the SMBH density in the local universe. These SMBHs hidden in stripped nuclei increase expected event rates for tidal disruption events and SMBH–SMBH and SMBH–BH mergers. The existence of numerous stripped nuclei with SMBHs are a direct consequence of hierarchical galaxy formation, but until now their impact on the SMBH density had not been quantified. Key words: galaxies: dwarf – galaxies: kinematics and dynamics – galaxies: nuclei – galaxies: star clusters: general – quasars: supermassive black holes 1. Introduction galaxy cluster. The presence of stripped nuclei can be used as a signpost of tidal disruption and merger processes. In the hierarchical galaxy formation framework, galaxies are One robust way to determine if a UCD is a galaxy nucleus is commonly accreted onto larger structures and their stellar ( by detecting central supermassive BHs. These have been content is stripped and distributed in the halo e.g., Steinmetz & fi ( 7 ) ( Navarro 2002; De Lucia & Blaizot 2007). Several studies have discovered in ve massive MM>´110 UCDs Seth et al. 2014; Ahn et al. 2017, 2018; Afanasiev et al. 2018), shown that it is possible to strip a galaxy in a larger cluster until ( ) only the central nuclear star cluster remains (Bekki et al. 2001, whereas no supermassive black holes SMBHs were found in ( 7 ) ( ) 2003; Drinkwater et al. 2003; Pfeffer & Baumgardt 2013; two lower mass MM<´110 UCDs Voggel et al. 2018 . Pfeffer et al. 2014, 2016). A candidate for those remnant These UCDs with nondetections could still be either a stripped stripped nuclei are ultra-compact dwarf galaxies (UCDs; galaxy nuclei that did not host a massive SMBH or a massive Minniti et al. 1998; Hilker et al. 1999; Drinkwater et al. GC. It was predicted that tidal stripping of galaxies would 2000). There is no physical definition of what constitutes a generate a population of SMBHs that are over-massive ’ ( UCD but commonly everything brighter and more massive compared to their host galaxy s mass Volonteri et al. 2008, 6 ) than ω Cen with 2×10 Me is categorized as a UCD (e.g., 2016; Barber et al. 2016 . Another cosmological simulation Mieske et al. 2008). As an alternative to their formation from also indicates that numerous stripped SMBHs above 6 stripped galaxy nuclei, UCDs may also be formed as massive 110´ M must live in the halo of Milky Way sized galaxies globular clusters (GCs; Mieske et al. 2004, 2012; Murray (Tremmel et al. 2018). 2009). The presence of an SMBH in the center of a UCD will increase It remains unclear how many UCDs are the stripped nuclei the dynamical mass estimates derived from integrated light of galaxies, but there is evidence that they are a mix of both dispersion measurements. This is because the black hole raises the GCs and stripped nuclei (Hilker 2006; Brodie et al. 2011;Da velocity dispersion of the stars near the center of the UCD and Rocha et al. 2011; Norris & Kannappan 2011). Theoretical when a light-traces-mass model is assumed, the dynamical mass simulations predict that stripped nuclei make up a majority of can be significantly overestimated. Such an elevated dynamical 7 UCDs at masses above MM> 10 , whereas lower mass mass has been observed in UCDs where on average the mass-to- UCDs are predominantly GCs (Pfeffer et al. 2014, 2016). light ratios (M/Ls) are elevated compared to simple stellar Because galaxy nuclei scale with their host mass (Ferrarese population models (SSP; Mieske et al. 2008, 2013; Strader et al. 7 et al. 2006), we can use stripped nuclei to determine the mass 2013). Especially at high masses above 110´ M most UCDs function of galaxies that were stripped; this would enable have elevated dynamical M/Ls. However, when we account for crucial constraints on the assembly history of a given galaxy or the dynamical effect of the SMBHs in the UCDs that host one, 1 The Astrophysical Journal, 871:159 (11pp), 2019 February 1 Voggel et al. their corrected stellar M/L ratios are all lower than expected from standard SSP models (Seth et al. 2014; Ahn et al. 2017, 2018). All of these massive UCDs with SMBHs are particularly metal-rich with approximately solar metallicity, where the predicted M/L of SSP models (Bruzual & Charlot 2003; Maraston 2005) increases sharply with increasing metallicity. M/Ls that are systematically lower than the SSP model prediction are also observed in lower mass GCs (Kruijssen & Mieske 2009; Strader et al. 2011; Mieske et al. 2013; Kimmig et al. 2015). Some works have found that GC M/L ratios have to be independent of metallicity (Kimmig et al. 2015; Baumgardt & Hilker 2018), while others even suggest decreasing M/L ratios at higher metallicities (Strader et al. 2011). In this paper we investigate whether comparing the dynamical masses of UCDs to an empirically measured M/L-metallicity relation instead of a theoretical relation is a better predictor of the presence of an SMBH in a UCD. Having a simple proxy for BHs in UCDs, such as the dynamical mass inferred from the integrated dispersion, would make it possible to constrain how many stripped nuclei exist in a galaxy cluster without the need for time-consuming adaptive optics observations that are not feasible for many UCDs. We use this new empirical M/L relation to predict the presence of SMBHs in individual UCDs as fi well as a rst estimate of their overall occupation fraction and Figure 1. Metallicity [Fe/H] of GCs and UCDs plotted against their measured their luminosity function. As a last step, we use the predicted dynamical M/LV. We include data from Baumgardt (2017) as blue points and amount of UCDs with SMBHs to estimate the increase in the clusters from Strader et al. (2011) as orange points. All clusters where the two- number density of SMBHs in the local universe, and quantify the body relaxation time is longer than log()t > 9.3 (gray crosses) were excluded fi ( ) effect this could have on tidal disruption and SMBH merger from the t. Four UCDs Seth et al. 2014; Ahn et al. 2017, 2018 for which the effect of a BH has been taken into account and their M/LV is only from the event rates. stellar population are shown in green. The dashed black line is the mean stellar population prediction for a 13 Gyr old population from Maraston (2005) and Bruzual & Charlot (2003), whereas the solid black line is our best-fit linear 2. Empirical M/L–Metallicity Relation relation and the dotted lines represent the intrinsic scatter 0 = 0.51. Standard stellar population models predict that old stellar populations have a M/L that increases with metallicity. In slope therefore ensures that we only include clusters with long / Figure 1 the dashed black line shows the expected M LV ratio relaxation times. for a 13 Gyr old stellar population, the same that was used in We then include the stellar M/L for four UCDs in green for ( ) ( ) V Mieske et al. 2013 and Dabringhausen et al. 2012 , which is which we have dynamical measurements of the SMBH mass (Seth ( ) a mean between the predictions from Maraston 2005 and et al. 2014;Ahnetal.2017, 2018). For these UCDs the dynamical ( ) ( ) Bruzual & Charlot 2003 for a Kroupa IMF Kroupa 2002 . effect of the BH has been accounted for by resolving the SMBH For a stellar population of solar metallicity, it predicts MLV ~ sphere of influence, and thus the M/L measurements are just for 4.5.
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